Bitmaps (File architecture): Difference between revisions
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<p>A | <p>A bitmap, as the name implies, is a series of on and off bits which are used to track a condition. These maps are used extensively throughout Model 204 to track sets of records or pages.</p> | ||
== The | == The workings of a bitmap == | ||
<p>The [[Page (File Architecture)|page]] size in Model 204 of 6184 bytes contains 6144 'usable' bytes (along with its 40 byte trailer). As such conditions of up to 49152 'things' (usually records but also pages) can be tracked on a single page.</p> | <p>The [[Page (File Architecture)|page]] size in Model 204 of 6184 bytes contains 6144 'usable' bytes (along with its 40 byte trailer). As such conditions of up to 49152 'things' (usually records but also pages) can be tracked on a single page.</p> | ||
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The most common use in Model 204 is to track sets of records: | The most common use in Model 204 is to track sets of records: | ||
* Inside both indexing types (hash and B-tree) | * Inside both indexing types (hash and B-tree) bitmaps are used | ||
* The [[#Table D (File Architecture) | * The [[#Table D (File Architecture) existence bitmap|existence bitmap]] which tracks which internal record numbers are in use. | ||
* During procedure execution, all found sets are held as | * During procedure execution, all found sets are held as bitmaps | ||
And, for [[Table E non x'100' (File Architecture)|Table E non FILEORG x'100']] files, large object available pages are tracked as well. | And, for [[Table E non x'100' (File Architecture)|Table E non FILEORG x'100']] files, large object available pages are tracked as well. | ||
== Manipulating | == Manipulating bitmaps == | ||
Once you have conditions represented as bitmaps, the manipulation ('anding' and 'oring') of these sets is straightforward and efficient: | Once you have conditions represented as bitmaps, the manipulation ('anding' and 'oring') of these sets is straightforward and efficient: | ||
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== Example == | == Example == | ||
<p>In this example (which looks at a | <p>In this example (which looks at a bitmap as used by an index), this is the beginning of the bitmap representing the ORD CHAR field value pair ''COLOR = BLUE'' for the 3rd segment of a file (which covers [[#Internal Record Number|IRNs]] 98304 to 147455).</p> | ||
:::[[File:Bit Map Example (File Architecture).jpg]] | :::[[File:Bit Map Example (File Architecture).jpg]] | ||
Revision as of 21:29, 18 December 2013
A bitmap, as the name implies, is a series of on and off bits which are used to track a condition. These maps are used extensively throughout Model 204 to track sets of records or pages.
The workings of a bitmap
The page size in Model 204 of 6184 bytes contains 6144 'usable' bytes (along with its 40 byte trailer). As such conditions of up to 49152 'things' (usually records but also pages) can be tracked on a single page.
Usages
The most common use in Model 204 is to track sets of records:
- Inside both indexing types (hash and B-tree) bitmaps are used
- The existence bitmap which tracks which internal record numbers are in use.
- During procedure execution, all found sets are held as bitmaps
And, for Table E non FILEORG x'100' files, large object available pages are tracked as well.
Manipulating bitmaps
Once you have conditions represented as bitmaps, the manipulation ('anding' and 'oring') of these sets is straightforward and efficient:
In code which has a find such as:
X: In filename FD COLOR = 'BLUE' STATE = 'NY' ...
What is happening (given the implied 'and') is that the sets of records for each criteria are retrieved, and then 'anded' (any bits 'on' on both, are 'on' on the resultant bitmap associated with the label 'X'.
Example
In this example (which looks at a bitmap as used by an index), this is the beginning of the bitmap representing the ORD CHAR field value pair COLOR = BLUE for the 3rd segment of a file (which covers IRNs 98304 to 147455).
.jpg)
Based in this, record 98304 does not have an occurrence of COLOR = BLUE, but records 98305 and 98306 do... and so on.